Hydrocarbon resources such as petroleum or natural gas have been produced by drilling a well (an oil well or gas well, collectively called “a well”) having a porous and permeable subterranean formation.
Among such hydrocarbon resources, shale oil and shale gas are petroleum and natural gas trapped in a shale formation located below 1000 to 2600 m underground. The shale oil and shale gas have been known to exist for a long time. However, the shale formed by compaction of fine particles exhibits poor fluid permeability and thus commercial production of shale oil and shale gas through a downhole have been considered difficult until recently.
However, in the 2000s, a horizontal drilling method (drilling method, in which a well is drilled horizontally along the shale formation to increase the contact area with gas), hydraulic fracturing (method, in which a fracturing fluid is introduced into a well at high pressure, crushing rocks by fluid pressure to form a fracture as a flow path of production fluid), and a seismic wave observation technique called micro seismic (technique of observing microearthquakes due to hydraulic fracturing to assess its performance and increase accuracy of the stimulation strategy) were established. Such techniques allow sufficient fluid permeability on the shale formation, and dramatically boost the production of shale oil and shale gas. Under circumstances where demands for energy are increasing worldwide, these oil and gas are expected as new energy and the market thereof has been rapidly growing.
Specifically, hydraulic fracturing, which is one of the well stimulation method is performed by the following processes such as: a perforation process in which, in a wellbore drilled in the vertical direction and/or the horizontal direction from the ground surface to the production reservoir (a subterranean formation, from which petroleum and the like are produced, for example, the shale formation, from which shale gas and the like are produced), the reservoir is perforated with a perforation gun; and a fracturing process in which a fracturing fluid containing proppants and the other chemicals is injected into the wellbore at high pressures to form and expand a fracture in the reservoir formation.
Additionally, after starting the production the reservoir may lose the fluid permeability due to the closure of the fracture with the passage of time, and stimulation of the productive layer is also performed to enhance the permeability again and to maintain efficient production rate. Stimulation of the reservoir is specifically performed by the following processes. That is, an isolation process of injecting a temporary diverting agent into a downhole, and temporarily plugging and isolating an existing fracture is performed. Then, as in a case of completing a new well, the perforation and a refracturing process are performed to form a new fracture. Thus, A successful refracturing operation restores well productivity to near original or even higher rates of production and extends the productive life of a well.
As the well-treatment fluid such as the fracturing fluid and the temporary diverting agent, various type of water-based, oil-based, and emulsion well-treatment fluids are used. Various chemicals such as proppants, viscosifying agents, acids for dissolving rocks and the like, and friction reducing agents are added to a solvent such as water.
However, in association with the recent growth of the market, environmental damage of chemicals added to the well-treatment fluid has being concerned. There is a movement to suspend the use of substances that may cause a large environmental damage and to switch to environmentally friendly alternatives.
For example, Patent Document 1 describes a thermothickening polymer used as a component for a fracturing fluid or the like. Patent Document 2 describes an example of using a polymer for a fracturing fluid, wherein the viscosity of the polymer increases in response to heat. Patent Document 3 describes using a thermothickening polymer as a fracturing fluid or a temporary diverting agent. Patent Document 4 describes using a thermothickening polymer having a cellulose chain to reduce unwanted water generation in a well and the like. Patent Documents 5 and 6 describe a fracturing fluid containing a surfactant and a thermothickening polymer. On the other hand, Patent Document 7 describes a non-ionic surfactant wherein aqueous solution viscosity of the non-ionic surfactant increases by heat, and also describes application to cosmetics and external use preparations.